Food preparation by critical path methods

OMEGA, The Int..rl of Mgmt S=i., Vol. 4, No. 2, 1976. Pergamon Press. Printe~t in Great Britain

Food Preparation by Critical Path Methods THROUGHOUT the US military system, large scale central food preparation facilities (CFPF) are replacing traditional company size units. This has been mandated by the available cost savings, improved service and quality uniformity [1]. One problem faced in the C F P F is the scheduling of food to be prepared. There exists a repeated menu cycle in which the particular foods and quantity are pre-determined; equipment is fixed. The bulk of the food is prepared and then frozen until the scheduled day of serving, at which time the food is heated and served. The basic question addressed here is the scheduling of the food preparation so as to be feasible relative to the constraints inherent in the facility and in the relevant food technological

considerations. Once feasibility is achieved, such considerations as minimum time schedules are of interest. As input to this scheduling problem, it was required to perform a critical path analysis on each food. at the appropriate batch size. These data were used to generate a schedule of equipment usage. An example is illustrated for preparation of a large batch o f baked lasagna. Table i below presents for each activity, the time required, equipment required, the 'early start time (ES)', 'late finish time (LF)', and 'slack time (S)', the latter three derived via standard critical path methods. Following Table I is the critical path diagram. We consider the results in Table 1 and

TABLE 1. ACTIVITIES, TIMES, EQUIPMENT

Activities A B C D E F G H I J K L M N O P Q R S T U V 14/

coat steam kettle add beef and heat drain fat open and crush tomatoes add tomatoes to meat and spice reduce heat and simmer adjust volume with water cool sauce in kettle transfer to panning area add water, salt, oil to kettle and boil add noodles and cook drain and rinse add oil and mix transfer to panning area combine eggs and cheese transfer to panning area transfer cheese to panning area pan lasagna heat oven to 325°F place lasagna in oven bake for 30 minutes remove from oven transfer to freezer

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t

ES

LF

S

5 10 5 I0 25 30 5 60 5 25 20 I0 5 5 5 5 5 30 30 10 30 I0 0

0 5 15 0 20 45 75 80 140 0 25 45 55 60 0 5 0 145 0 175 185 215 225

5 15 20 20 45 75 80 140 145 105 125 135 140 145 140 145 145 175 175 185 215 225 225

0 0 0 10 0 0 0 0 0 80 80 80 80 80 135 135 140 0 145 0 0 0 0

Equipment kettle 1 (K~) Kt, agitator KL

VCM K~, agitator Kt

Kt K~, agitator -kettle 2, (Kz) K,, agitator

Kz K,_ -mixer ---oven oven oven oven --

Omega, Vol. 4, No. 2 r

E

~

F

FIG. 1. Critical path diagram. develop an 'Equipment usage summary.' consider the first kettle system (Kt). By noticing that all the activities requiring Kt are on the critical path (no slack), the time requirement for Kt is not flexible (assuming we wish to maintain the minimum time for total project completion), and is required during time interval 0-140. (We arbitrarily assume initially, for each food, a zero start time). The second kettle system (/('2) is used during activities J, K, L, M, which comprise a total of 60 rain. However, there is a lot of flexibility concerning which 60 min. By looking at the early start time of activity J, and the late finish time of activity M, we find that we need K_, for any 60-rain interval between 0-140. The agitator summary is a little more complicated. When used with K~, there is no choice and the required times are 5-15, 20-45, 80-140. However, when used with Kz (activity K) it is required for 20 min, but it must be the 20 rain selected for activity K once the 60 rain for/£2 is selected. The VCM is required for only 10 rain, but it must be sometime in the 0-20 interval. The mixer is required for only 5 rain and that 5 rain can be anywhere in the 0-135 time interval. In both cases the time interval was determined by looking at the early start and late finish of the one activity using the particular piece of equipment. Finally, we consider the oven. According to the analysis, it would be indicated that we require the oven from 175-225 with no choice (activities T, U, V on the critical path), and any 30-rain period in the interval 0-175. However, this conclusion would be super-

ficial. Clearly, if you heat the oven during any 30 min period before 145-175, it would not be available for (most) other products because the temperature is specific for the food under consideration. Thus we would conclude that the oven usage is essentially 145-225. (It is true that there may be some gain by producing at the same times, foods which require the same oven temperature. This fact was recognized but it was then decided not to consider it explicitly for the moment, since it was judged to be a factor to be built in later). The purpose of generating these equipment usage summaries was to use them as input on the computer to generate via simulation a series of feasible production schedules. However, another purpose of the critical path analysis and equipment usage summary was to set up a complete program manual for the workers to follow during the preparation process. This is extremely useful, especially when considering that each day may find different people performing certain tasks in the CFPF.

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REFERENCES 1. BUST~AD R et al. (1972) A Proposed Modern Food Service System for Fort Lewis, US Army Natick Laboratories Technical Report, 73-10. D r PD BEGGER

(December 1975) School of Management Boston University 212 Bay State Road Boston Massachusetts 00215 USA